Six months in to the first full election cycle in which Super PACs will play a role, the groups have combined to raise more than $26 million and are entering the second half of the year with about $23 million on hand.

Ninety-one Super PACs reported their receipts and disbursements to the FEC for all or part of the first six months of 2011. But the fundraising among the committees was dominated by a handful of groups, with the top five accounting for $22 million — about 83 percent — of the total.

Unlike traditional political action committees, Super PACs are allowed to raise unlimited amounts from individuals, labor unions, corporations and other political committees (traditional PACs can raise only limited amounts from individuals and other PACs). However, Super PACs must work independently of candidates and cannot give money to them or coordinate spending with them. Disbursements made in support or opposition of a candidate but without coordinating with the candidate or the candidate's opponent are known as independent expenditures.

Super PACs spent more than $60 million on independent expenditures in the 2010 cycle, though they didn't exist until about four months before the election, when an FEC advisory opinion outlined the process for registering such a group. The groups have spent nearly $4 million on independent expenditures so far this year, much of it in special elections for two House seats — the California 36th and the New York 26th.

Many Super PACs' first reports to the FEC this year were due Sunday. Others file more frequently, and any group that spends money in an election must file a report after Election Day.

Here's a look at where some of the Super PACs with the biggest fundraising numbers stood for the first half of the year (download data):

Restore Our Future brought in the most money by far of any Super PAC and, with only $22,000 in disbursements so far, has most of that money left to spend. Unlike many of the other groups, though, it was formed to support a single presidential candidate, Mitt Romney, and is expected to spend big during primary season. Four donors gave $1 million each: hedge-fund manager John Paulson; Eli Publishing Inc. and F8 LLC, which both have the same address in Provo, Utah; and W. Spann LLC. Contributors who gave $500,000 included Paul Edgerly of Bain Capital — which Romney co-founded — and his wife; hotel executives J.W. Marriott and Richard Marriott; and homebuilder Bob Perry.

American Crossroads, which counts former George W. Bush aide Karl Rove among its advisers, spent more on independent expenditures than any other Super PAC in the 2010 election cycle. It started the year with about $760,000 and has brought in nearly $4 million more. The group spent about $690,000 on independent expenditures in the special election for New York's 26th District House seat. The Reporting Group wrote previously about American Crossroads' contributors, who include former Univision CEO Jerry Perenchio, TRT Holdings CEO Robert Rowling and Texas homebuilder Bob Perry. In the two weeks between its last report, filed after the New York 26th election, and the end of the filing period, the group's biggest contributor was John Templeton Jr., who gave $100,000.

Priorities USA Action, which was started by former aides to President Obama, received its largest contribution — $2 million, accounting for about 63 percent of all the money it brought in — from Dreamworks CEO Jeffrey Katzenberg. Katzenberg is also among the individuals who have "bundled" large amounts from other contributors for Obama. Other large donors to the group include contributors whose names also appear on other Super PAC fundraising lists: labor union SEIU ($500,000) and Fred Eychaner ($500,000). Priorities USA Action has already spent about $96,555 opposing Republican presidential candidate Mitt Romney.

American Bridge was begun by Democratic operative David Brock in an attempt to offset the effects of heavy spending by outside Republican groups. The committee reported has brought in about $1.5 million so far, but also reported almost $200,000 in debts and obligations — mostly for office-related expenses and legal costs. Among the contributors to the group were insurance executive Peter Lewis, who gave $200,000; producer Stephen Bing, who gave $150,000; and the PAC of labor union SEIU, which gave $100,000.

Majority PAC, which supports Democratic candidates for Senate, reported two contributions of $250,000 so far this year — one from labor PAC SEIU COPE and one from producer Stephen Bing (both also gave money to American Bridge 21st Century). The group also received $150,000 each from Las Vegas retiree Guy Gundlach and William E. Little Jr. of New York, whose occupation is also listed as retired. The group has already spent about $65,000 on independent expenditures this year.

House Majority PAC, which supports Democratic House candidates, spent heavily — about $370,000 — in the New York 26th District House race earlier this year. In its first report of contributions to the FEC after that election (which the Reporting Group wrote about here), the committee listed among its biggest donors the labor unions American Federation of State, County and Municipal Employees ($200,000) and the Communications Workers of America ($50,000) and individuals Fred Eychaner ($100,000) and George Soros ($75,000). In its latest filing, which covers the last two weeks of June, House Majority PAC reported as its only contributor another labor union, SEIU, whose PAC gave the committee $185,000.

Other Super PACs, while not among the top fundraisers, are also worth taking note of:

Republican Super PAC was started by campaign-finance lawyer James Bopp with the goal of getting candidates and party committees to solicit unlimited contributions for it. In response, two Super PACs that support Democratic candidates — Majority PAC and House Majority PAC — asked the FEC whether such solicitations are allowed and whether they, too, could have candidates solicit unlimited amounts on their behalf. The FEC ruled that candidates and party officials could solicit only limited amounts for Super PACs — up to $5,000 per contributor. In an e-mail posted on the Election Law Blog in response to a comment that the group was doing "not so super", Bopp wrote, "Interesting take, but not really accurate. The RSPAC is a general election federal PAC and is not involved in primaries. In addition, we do not raise funds on our own, like other Super PACs, but rely on Republican activists, state party officials and candidates to do this with their maxed out donors. We do not expect to raise any funds for the 2012 general election until well into next year." All of the contributions reported in Republican Super PAC's most recent filing were from the group's three founders.

The $193,000 raised by this Super PAC came from just seven donors, and the four largest of those donors accounted for 96 percent of the total raised. The biggest contributor was Harold Simmons, who gave $100,000 to the Super PAC supporting Texas Gov. Rick Perry, who is considering a presidential run. According to the Huffington Post, Simmons and his wife gave $600,000 to Perry's re-election campaign in 2010.

The biggest contribution to ChristinePAC, which was begun in January by former Delaware Senate candidate Christine O'Donnell, was $50,000 from O'Donnell's Senate campaign committee, Friends of Christine O'Donnell. While Super PACs may not give money to a campaign committee, there is no rule forbidding a campaign committee from giving money to a Super PAC.

Like ChristinePAC, Our Voice PAC was started by a former Senate candidate — Sharron Angle, who lost the race for the Nevada seat to Harry Reid last year. Also like ChristinePAC, the biggest contributor to the group was a campaign committee — Friends of Sharron Angle. The campaign committee gave $7,500 to Our Voice PAC and was the only itemized contribution listed, meaning that the remaining $5,440 came in contributions of under $200 each.

All of the receipts reported by the Citizens United Super PAC were in-kind contributions for administrative support from Citizens United. It was a court case brought by Citizens United that led the Supreme Court to rule that corporations could spend unlimited amounts on independent expenditures. This was followed by a decision by a lower court that individuals, corporations and labor unions could contribute unlimited amounts to committees that did not coordinate with candidates.

Turn Right USA is best known for a controversial internet advertisement opposing Janice Hahn in the special election for California's 36th District House seat. It reported no itemized receipts (meaning all contributions were below $200) and ended the second quarter with about $8,000 in debt.